Different chemical substances absorb varying frequencies of the visible spectrum. Colorimeters rely on the principle that the absorbance of a substance is proportional to its concentration i.e., a more concentrated solution gives a higher absorbance reading. A filter in the colorimeter selects the frequency of light which the solute absorbs the most, in order to maximise the accuracy of the experiment. Note that the colour of the absorbed light is the 'opposite' of the colour of the specimen, so a blue filter would be appropriate for an orange substance. Sensors measure the amount of light which has passed through the solution and a display reads the amount absorbed.
A quantitative reading for the concentration of a substance can be found by making up a series of solutions (of known concentration) of the chemical under study, and plotting a graph of absorbance against concentration. By reading off the absorbance of the specimen substance, a value for its concentration is known.